Thermodynamics

96 | ThermodynamicsRoom air29°CFIGURE 2–75Q condQ convHeat transfer from the persondescribed in Example 2–19.20°CQ radthat the experimentally determined value for the rate of convection heattransfer in this case is 6 W per unit surface area (m 2 ) per unit temperaturedifference (in K or °C) between the person and the air away from the person.Thus, the rate of convection heat transfer from the person to the air in theroom is, from Eq. 2–53,Q # conv hA 1T s T f 2 16 W>m 2 # °C2 11.6 m 2 2129 202 °C 86.4 WThe person will also lose heat by radiation to the surrounding wall surfaces.We take the temperature of the surfaces of the walls, ceiling, andthe floor to be equal to the air temperature in this case for simplicity, butwe recognize that this does not need to be the case. These surfaces maybe at a higher or lower temperature than the average temperature of theroom air, depending on the outdoor conditions and the structure of thewalls. Considering that air does not intervene with radiation and the personis completely enclosed by the surrounding surfaces, the net rate of radiationheat transfer from the person to the surrounding walls, ceiling, andthe floor is, from Eq. 2–57,Q # rad esA 1T 4 s T 4 surr2 10.952 15.67 10 8 W>m 2 # K 4 211.6 m 2 2 3129 2732 4 120 2732 4 4K 4 81.7 WNote that we must use absolute temperatures in radiation calculations. Alsonote that we used the emissivity value for the skin and clothing at room temperaturesince the emissivity is not expected to change significantly at aslightly higher temperature.Then the rate of total heat transfer from the body is determined by addingthese two quantities to beQ # total Q # conv Q # rad 86.4 81.7 168.1 WThe heat transfer would be much higher if the person were not dressed sincethe exposed surface temperature would be higher. Thus, an important functionof the clothes is to serve as a barrier against heat transfer.Discussion In the above calculations, heat transfer through the feet to thefloor by conduction, which is usually very small, is neglected. Heat transferfrom the skin by perspiration, which is the dominant mode of heat transferin hot environments, is not considered here.SUMMARYThe sum of all forms of energy of a system is called totalenergy, which consists of internal, kinetic, and potentialenergy for simple compressible systems. Internal energy representsthe molecular energy of a system and may exist insensible, latent, chemical, and nuclear forms.Mass flow rate ṁ is defined as the amount of mass flowingthrough a cross section per unit time. It is related to the volumeflow rate V . , which is the volume of a fluid flowingthrough a cross section per unit time, bym # rV # rA c V avg